Known web transporting devices comprise a fixed driven transport roller and at least one narrow engagable nip roller. These narrow engagable nip rollers are inconvenient to use in folders. With each new printing job, the width of a ribbon of web material may change. If so, the narrow nip rollers must be realigned so that they contact both edges of the ribbons. This involves loosening bolts that are used to clamp the nip rollers onto a shaft and sliding nip collars to which each nip roller is attached to the correct position, and then retightening the bolts. This is time consuming and involves considerable machine down-time.
Furthermore, since the nip rollers do not need to operate at a high compressive force, any vibratory impacts between them will tend to separate the nip rollers from the fixed driven transport roller. This leads to ribbon tension variations. In the past, the nip actuation force was increased but the nip rollers' nip force was not because a mechanical stop device was incorporated to determine the roller-to-roller gap. The setting of the mechanical stop is inconvenient and time consuming and must usually be done for each new printing job.
Several attempts have been made to correct these problems. British Patent No. 1,457,989 and U.S. Pat. No. 3,083,887 disclose web transporting devices which are directed to controlling the tension of the web material as it passes through the web transport device.
British Patent No. 1,457,989 discloses a carrier mounted in supports which is moved by two cylinders which are supplied with a pressure medium via a common pressure-medium connection. The force of the pressure is selectable via a control. The mutual connection of the individual chambers of the cylinders effects a displacement of fluid volume from cheer to chamber as a result of volume variations, the position of a lever mounted on one side being thereby stabilized, and the web tension being accordingly held constant.
U.S. Pat. No. 3,083,887 discloses a device having a cylinder subjected to a pressure medium which deflects a lever mounted on one side, in order thereby to increase and decrease, respectively, the belt tension of a drive via a roller.
A disadvantage of these solutions is that they do not provide a sensitively variable or meterable engagement of the rollers due to the long lever arms. On the other hand, due to the use of conventional shock-absorbers and the use of a compound chamber system, respectively, the damping characteristic is severely restricted or limited.
A further disadvantage of these solutions is that they create too much tension on the web material causing it to wrinkle.
One solution to these problems has been to provide an actuation means for applying a variable force of engagement of a engagable full width nip roller with a fixed driven transport roller. Such a device draws essentially wrinkle-free webs which may be formed of several web layers under constant web tension through a folder. This permits the formation of an exact longitudinal fold in the nip between the engagable full width nip roller and the transporting roller. A damping means acting in parallel with the actuating means on the full width nip roller stabilizes the position of the engagable transport roller and prevents or limits a build-up of vibrations.
Although this solution solves many of the above-mentioned problems in the prior art devices, it has a few drawbacks. When nip rollers of this type become damaged or worn, the whole nip roller must be removed. Because the nip roller is full width it is cumbersome and heavy, thus making removal a difficult and time consuming task. Also, when nip rollers of this type become worn they must be rebuilt which involves considerable machine down time.